Development and performance evaluation of a diurnal passive radiative cooling system for application in buildings
Date
2024-02
Authors
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Publisher
Federal University of Technology, Owerri
Abstract
Space cooling for thermal comfort consumes significant amount of energy, and this has continued to be a major global challenge. However, space cooling needs are increasing in the tropics due to increase in energy intensive lifestyles, population increases, and incessant increases in ambient temperature. Global warming and climate change issues associated with convectional energy usage are increasingly informing investments in sustainable and clean energy systems. Accordingly, the development and performance evaluation of a diurnal passive radiative cooling system for space cooling application in buildings are hereby presented. Two diurnal passive radiative coolers using dual-layer mesoporous polyethylene aerogel and ultra-white barium sulphate on silver substrates as selective reflective surfaces were designed and fabricated. This test rig is a rectangular box with two compartments in which the coolers are installed for investigation. The top of the test rig is covered with a low density polyethylene windscreen, which is transparent to radiation in all wavelengths. Thermocouples are inserted underneath and outside the coolers to record the temperatures of the coolers and the ambient respectively. Extensive field investigations were conducted with these photonic coolers under direct solar irradiance in three different locations under Nigerian climatic conditions, namely, Abuja, Afikpo, and Owerri. The experimental results for a 24- hour test show that the diurnal passive radiative system achieved significant cooling performances with sub-ambient temperatures of about 3-5, 3-6, and 2-6 oC during the daytime and 1-2, 0-1, and 0.5-2 oC at night, for Abuja, Afikpo, and Owerri respectively for Mesoporous polyethylene aerogel cooler. For the barium sulphate cooler, sub-ambient temperatures of 2-3, 1.5-3.5, and 1-3.5oC during the daytime and 0-1.5, 0-0.5, and 0-1 oC at night were reported. Average cooling powers of 108.84, 84.99, and 101.06W/m2 were recorded in Abuja, Afikpo, and Owerri respectively with the mesoporous polyethylene Aerogel, while 120.29, 89.81, and 106.74W/m2 were recorded with the barium sulphate in Abuja, Afikpo, and Owerri, respectively, during the investigation. Also, a theoretical model was developed and used to numerically predict the thermal performances of the coolers in six locations, namely, Abuja, Yola, Afikpo, Ado-Ekiti, Kaduna, Owerri, and Warri, representing the different geopolitical zones in Nigeria. The model equations were discretized using finite element numerical scheme for ease of implementation on a digital computer. The system behaviour was simulated with the developed model using a code written with FlexPDE Finite Element Model Builder and Numerical Solver Version 7.12. The numerical results obtained from the models were compared with experimental results and other similar data from the literature. The deviations of the model and experimental temperatures were recorded as ±2.73oC for Mesoporous polyethylene aerogel and ±1.81oC for barium sulphate. Thus, for the numerical performance evaluation, the model results closely matched experimental results with total standard uncertainties of 0.64 for Mesoporous polyethylene aerogel and 0.37 for barium sulphate. These deviations are less than 1, which is a necessary condition for model accuracy. Sensitivity analyses show that the optimal performance parameters are 15mm, 30o , and 5, for the cooler thickness, orientation and number of windscreens, respectively. Therefore, further material development, extended field investigation, and possible deployment of passive radiative cooling system for use in residential buildings are recommended.
Description
This thesis is for the award of Doctor of Philosophy (PhD.) Degree in Mechanical Engineering, Energy and Power Engineering Option
Keywords
Thermal comfort, sub-ambient cooling, diurnal radiative cooling, numerical simulation, thermal insulation, solar absorption, Department of Mechanical Engineering
Citation
Okoro, H. O. (2024). Development and performance evaluation of a diurnal passive radiative cooling system for application in buildings [Unpublished Doctoral Thesis]. Federal University of Technology, Owerri, Nigeria